U.S. Pat. No. 5,115,639 discloses an engine air/fuel control system responsive to exhaust gas oxygen sensors positioned both upstream and downstream of a catalytic converter.
It is also known to provide an indication of converter degradation by comparing the amplitude output of the downstream sensor to the upstream sensor. In another type of system, converter degradation is determined by calculating a ratio of switching frequencies between the upstream and downstream sensors. The downstream sensor amplitude and its switching frequency increase as converter efficiency decreases.
The inventors herein have recognized numerous problems with the above approaches to monitoring converter efficiency. With respect to amplitude comparison, exhaust gases from each of the combustion chambers may not be fully mixed in the exhaust manifold. Consequently, there may be variations in amplitude output of a downstream sensor which are caused by lack of exhaust gas mixing rather than converter degradation. Another problem recognized by the inventors, is that sensor amplitude output may become clipped when the engine is operating significantly rich or lean of stoichiometry. Accordingly, amplitude comparisons may not yield accurate determinations of converter efficiency.
The inventors have also discovered significant problems with respect to those systems which utilize comparison of sensor switching frequencies. If the engine air/fuel feedback control loop is in a transient state during a converter test cycle, there may be insufficient switching of the sensors to fully test converter efficiency. Further, both types of prior systems (sensor switching systems and sensor amplitude comparison systems) do not test converter efficiency over the full range of possible exhaust gas flow rates. Converter efficiency may, therefore, not be fully tested.